RemotePostWebformHandler does not cast zero to false

Problem/Motivation

RemotePostWebformHandler gives the option to post the data as JSON and also cast them. If we choose to do so Checkboxes are casted to booleans.

Very early in the getRequestData() method there is a check which ignores empty elements:

...
    foreach ($data as $element_key => $element_value) {
      if (empty($element_value)) {
        continue;
      }
...

This works in the most of the cases and ignores the empty values including the 0.

In the case of a checkbox though, 0 is not and empty value and should not be ignored. If we do so 0 will properly be casted to false

Steps to reproduce

1. Create a for with a checkbox
2. Attach the RemotePostWebformHandler
3. Set set the submission method to POSTand enable Cast posted element value and custom data

The try to submit the form and monitor the posted JSON. The checkbox value is posted as true when selected and 0 when not.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes